These varactors are semiconductor diodes with a capacitance that is dependent on the applied voltage and are commonly used in modern communication systems. Integrated circuits often include varactors (“variable reactors”). Varactors provide a voltage controlled capacitive element that has a variable capacitance based on the voltage expressed at the terminals and a control voltage. Metal oxide semiconductor (MOS) varactors may have a control voltage applied to a gate terminal that provides a control on the capacitance obtained for a particular voltage applied on the remaining terminals of the device.
Because a varactor is based on a reverse biased P-N junction, the terminals are typically biased such that no current flows across the P-N junction, thereby forming a capacitor. However, varying the bias on the gate of a MOS varactor causes the formation of a depletion or an accumulation region under the gate, changing the current flow through the varactor. The effective capacitance obtained is thus variable, and, voltage dependent. This makes the varactor useful as a voltage controlled capacitor. Varactors are particularly useful in oscillators, RF circuits and in conventional communication technologies to generate given frequencies used as input signals.
Two types of conventional MOS varactors are often used. One type is an n-MOS accumulation-type varactor that has a simple implementation. However, in an n-MOS accumulation-type varactor, a parasitic diode is turned on when Vcontrol<0 because the substrate is shorted to ground. This results in a low Q factor during half of the tuning range. The other type is an inversion MOS varactor, which has a parasitic diode that is always reverse biased, preventing leakage to the substrate. However, an inversion MOS varactor has a narrow tuning range.
The conventional varactor applications use control systems such as phase-locked loops (PLLs) to generate output signals in response to a given input signal. PLLs are essential circuits used for a variety of timing, synchronization, and signal processing functions in a range of electronic applications. An important application of PLLs is in telecommunication and radar systems, where they are used to generate the carrier frequencies, local oscillator frequencies, and intermediate frequency signals. The present trend in wireless communication systems is towards the use of all-digital PLLs (ADPLLs), which offer the advantages of smaller chip size, better scalability, and extensive re-configurability compared to traditional, analog, PLLs.